1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device that uses a thin-film semiconductor having crystallinity. In particular, the invention relates to manufacture of an insulated-gate thin-film transistor (TFT).
2. Description of the Related Art
In recent years, the techniques of forming a TFT using a crystalline silicon film (particularly a film made of a material called polysilicon film) on a substrate having an insulative surface have been developed. A TFT using such a material is advantageous over a TFT using an amorphous silicon film in being capable of high-speed operation.
For this reason, extensive studies are now being made on the monolithic panel in which a pixel matrix circuit and driver circuits are formed on the same substrate as well as the system-on-panel structure in which signal processing logic circuits (memories, amplifiers, a CPU, etc.) are additionally formed in an integral manner. For example, the driver circuits and the logic circuits are formed as a composite circuit in which a CMOS circuit (inverter circuit) that is a complementary combination of an n-type TFT and a p-type TFT is used as a basic circuit.
The TFT that constitutes such a variety of circuits is a switching element that is turned on when a particular voltage (called a threshold value or a threshold voltage) is applied to the gate electrode and that is rendered off when a voltage lower than the particular voltage is applied. Therefore, precise control of the threshold voltage is very important for correct operation of a circuit.
However, there may occur an event that the threshold voltage of a TFT shifts to the positive or negative side due to indefinite factors in a manufacturing process, such as influences of mobile ions that have been introduced by pollution and a difference in work function or interface charge in the vicinity of the gate of the TFT.
Such a shift of the threshold voltage causes adverse effects such as impairing the switching element function and increasing the power consumption. Although the pollution-induced factors can be removed by, for instance, improving the process, the factors caused by a work function difference or the like are determined by the materials used and hence are difficult to remove in some cases.
The channel doping has been proposed to remove the latter factors. The channel doping is a technique of controlling, i.e., intentionally shifting the threshold voltage by adding an impurity element (typically, P, As, or B) that imparts one conductivity type to at least the channel forming region of a TFT. To control the threshold voltage to a desired value, it is necessary to control the addition amount of the impurity element very precisely.
The impurity element may be added by mixing it into a gas for forming an amorphous silicon film or a crystalline silicon film or by performing ion implantation or the like after crystallization. Further, the impurity element may be added selectively, i.e., only to a portion, to become a channel forming region, of a crystalline silicon film that has been shaped into an island-like pattern.
After concentrated studies for obtaining superior TFT characteristics, the present inventors invented a crystalline silicon film having much superior crystallinity. Conditions necessary for forming this crystalline silicon film will be described below briefly.
First, an amorphous silicon film is formed on a highly heat resistant substrate (for instance, a quartz substrate) and then crystallized by utilizing the technique disclosed by the present inventors in Japanese Unexamined Patent Publication No. 7-130652, which is a technique of adding a catalyst element (typically nickel) for accelerating crystallization to an amorphous silicon film and then crystallizing it by a heat treatment. The disclosure thereof is incorporated herein by reference.
After a crystalline silicon film has been obtained, the catalyst element is gettered by performing a heat treatment in an atmosphere containing a halogen element. This gettering step utilizes the metal element gettering effect of the halogen element. To obtain a sufficient gettering effect of the halogen element, it is preferable that the heat treatment be performed at more than 700xc2x0 C.
In the gettering step, the catalyst element remaining in the crystalline silicon film is combined with, i.e., gettered by, the halogen element to become a volatile halide which escapes into the air. The catalyst element is thus removed. As a result of the catalyst element gettering step, the concentration of the catalyst element in the crystalline silicon film is reduced to less than 1xc3x971017 atoms/cm3 (preferably less than the spin density). In this specification, the impurity concentration is defined as the minimum value of measurement values obtained by secondary ion mass spectroscopy analysis.
A crystalline silicon film that is formed in the above-described manner has a feature that it is a crystal structural body as a collection of a plurality of rod-like or flat-rod-like crystals and microscopically the growth directions of the respective rod-like crystals are aligned in a particular direction. Further, the crystallinity inside the crystals has been greatly improved by the heat treatment of the gettering step.
However, in experimentally producing various kinds of TFTs by using crystalline silicon films of the above kind, the present inventors have found that a serious problem occurs in applying the above-mentioned channel doping. This is an phenomenon that in removing a catalyst element in the gettering step, an impurity element (B, P, As, or the like) that has been added to the surface layer and its vicinity of the crystalline silicon film is exhausted. This phenomenon is reported in IBM Technical Disclosure Bulletin, Vol. 1, No. 5, 1973. The disclosure thereof is incorporated herein by reference.
Since the concentration of the impurity element in the region (in the vicinity of the surface of the active layer) where a channel is to be formed is much reduced, the intended effect of the channel doping is not obtained, which makes it impossible to control the threshold voltage precisely.
The above problem was first recognized when the conventional channel doping was applied to the above-described method of forming a crystalline silicon film; no one recognized the problem before that. An object of the present invention is therefore to solve the above entirely new problem that no one has recognized.
Specifically, an object of the invention is to provide a technique for performing a heat treatment in an atmosphere containing a halogen element without exhausting an impurity element such as phosphorus or boron existing in the vicinity of the surface of a crystalline silicon film.
According to one aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of forming an amorphous silicon film containing a group-13 or group-15 impurity element; converting the amorphous silicon film into a crystalline silicon film by performing a heat treatment; and performing a heat treatment in an atmosphere containing a halogen element and a compound gas that contains the impurity element.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of holding, adjacent to an amorphous silicon film containing a at group-13 or group-15 impurity element, a catalyst element for accelerating crystallization of the amorphous silicon film; converting at least part of the amorphous silicon film into a crystalline silicon film by performing a heat treatment; and removing or reducing a concentration of the catalyst element from the crystalline silicon film by performing a heat treatment in an atmosphere containing a halogen element and a compound gas that contains the impurity element.
According to a further aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of adding a group-13 or group-15 impurity element to an amorphous silicon film; holding a catalyst element for accelerating crystallization of the amorphous silicon film adjacent to the amorphous silicon film; converting at least part of the amorphous silicon film into a crystalline silicon film by performing a heat treatment; and removing or reducing a concentration of the catalyst element from the crystalline silicon film by performing a heat treatment in an atmosphere containing a halogen element and a compound gas that contains the impurity element.
In the invention, in gettering a catalyst element in an atmosphere containing a halogen element, a compound gas containing a group-13 or group-15 impurity element (for threshold voltage control) is mixed into the processing atmosphere. (Preferably, a compound gas of the halogen element and the impurity element is mixed.)
As a result, a chemical equilibrium state for the impurity element is established between the processing atmosphere and the surface to be processed. With this measure, the chemical reaction involving the impurity element near the surface of the silicon film can be suppressed, whereby the impurity element for threshold voltage control can effectively be prevented from being removed from the silicon film.
Typical examples of the impurity element for threshold voltage control (channel doping) are a group-13 element of boron (B) and group-15 elements of phosphorus (P) and arsenic (As). Group-13 elements of aluminum (Al) and gallium (Ga) and a group-15 element of antimony (Sb), etc. may also be used. The selection among those elements is made in accordance with the conductivity type of an intended TFT and the threshold voltage shifting direction (positive or negative side).
Examples of the compound gas containing the impurity element for threshold voltage control are diborane (B2H6), boron trifluoride (BF3), boron trichloride (BCl3), aluminum trichloride (AlCl3), and gallium trichloride (GaCl3) each of which include a group-13 element, and phosphine (PH3), phosphorus trichloride (PF3), phosphorus trichloride (PCl3), arsine (AsH3), arsenic trifluoride (AsF3), arsenic trifluoride (AsCl3), stibine (SbH3), and antimony trichloride (SbCl3) each of which include a group-15 element.